Use transparent for invisible borders of page navigation rather than white

This makes the effect better when you override page background color,
like we do on mediawiki.org e.g. on manual pages like
<https://www.mediawiki.org/wiki/Manual:Installation_guide>.

Change-Id: Ie863d11b19a56ab6cf75ff4b8dfcd2d74b69ce1d

resources/screen-common.less

@@ -212,7 +212,7 @@ div.suggestions {
 	}
 	li {
 		margin: 0.25em 0 1em;
-		border-bottom: solid 3px @background;
+		border-bottom: solid 3px transparent;
 
 		&:hover {
 			border-bottom-color: @grey;